The
River Lagan (;
Ulster Scots:
Lagan Wattèr) is a major river in
Northern Ireland which runs 53.5 miles (86 km) from the
Slieve Croob mountain in
County Down to
Belfast where it enters
Belfast Lough, an inlet of the
Irish Sea. The
River Lagan forms much of the border between
County Antrim and
County Down in the east of
Ulster. It rises as a tiny, fast-moving stream near to the summit of Slieve Croob; Transmitter Road runs nearby. From here it continues on its journey to Belfast through
Dromara,
Donaghcloney and
Dromore. On the lower slopes of the mountain, it is joined by another branch from Legananny (Cratlieve) Mountain, just opposite Slieve Croob. At Dromara, about four miles from its source, its height above the sea is 390 ft (119m). As the river continues on its journey to Belfast, it turns east to
Magheralin into a broad plain between the plateaus of Antrim and Down.